fopen.3: Document ",ccs=string" feature of 'mode' for fopen()/freopen()

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/fopen.3

@@ -300,9 +300,32 @@ to
 .BR EEXIST .
 This flag is ignored for
 .BR fdopen ().
-.\" FIXME document /,ccs= charset/
-.SH BUGS
-In versions of glibc before
+.PP
+In addition to the above characters,
+.BR fopen ()
+and
+.BR freopen ()
+support the folloing syntax
+in
+.IR mode :
+
+.BI "    ,ccs=" string
+
+The given
+.I string
+is taken as the name of a coded character set and
+the stream is marked as wide-oriented.
+Thereafter, internal conversion functions convert I/O
+to and from the character set
+.IR string .
+If the
+.BI ,ccs= string
+syntax is not specified,
+then the wide-orientation of the stream is
+determined by the first file operation.
+If that operation is a wide-character oepration,
+the stream is marked wide-oriented,
+and functions to convert to the coded character set are loaded.
 .SH "SEE ALSO"
 .BR open (2),
 .BR fclose (3),